クロム-ニッケル・ステンレス鋼の組織と諸性質におよぼすチタン, ニオブの影響

抄録

Investigations were made on the effects of Ti and Nb additions on structures of Cr-Ni stainless steels and on the effects of the structures on mechanical properties and corrosion resistance.
Experimental results obtained were as follows: 1) The specimens containing Ti were hardened by the formation of σ phase from δ ferrite and the precipitation of Ni₃Ti. Those containing Nb were hardened only by the formation of σ phase from δ ferrite.
Two peaks were observed in as-aged hardness versus time curves for the specimens containing Ti when they were solution treated at lower temperature.
2) The chromium equivalences of Ti and Nb as a ferrite former were about 35 and 3'1 respectively. In the specimens contaning the largest amounts of Ti and Nb, Laves phase existed in the neighbourhood of δ ferrite. The chromium content of σ phase became low in specimens containing Nb as in specimens containing Ti and Mo.
3) The degree of the solution hardening by Ti and Nb was considerably great. The effect of the solution hardening by Ti on tensile strength was larger than that by Nb. In specimens containing Ti, tensile strength increased with increasing Ti content at test temperatures below 700°C.
4) Creep rupture strength reached minimum at certain contents of Ti and Nb. This was due to the combined effects of the solution hardening and the increment of the amount of δ ferrite with increasing contents of the alloying elements.
5) The corrosion resistance to H₂SO₄ also reached minimum at certain contents of Ti and Nb, which was similar to the relationship between creep rupture strength and alloying content, as described above.